Question: $ { {1} \times \left[\begin{array}{rr}{1} & {-2} \\ {1} & {2} \\ {0} & {-1}\end{array}\right] = {?} } $
To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{rr}{1}\times{1} & {1}\times{-2} \\ {1}\times{1} & {1}\times{2} \\ {1}\times{0} & {1}\times{-1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{1} & {-2} \\ {1} & {2} \\ {0} & {-1}\end{array}\right]}$